1. Field of the Invention
The present invention relates generally to space-based navigational systems, and in particular, to a space-based lever arm correction system applicable to navigational systems employing spot beams.
2. Description of the Related Art
The Global Positioning System (GPS) is a satellite system that transmits navigation signals that are received by ground-based GPS receivers and used to determine the position of the GPS receiver to a high degree of accuracy. GPS currently provides standard service to commercial receivers, and a higher accuracy service to military receivers authorized to receive such signals.
In current systems, the GPS navigation signal is transmitted via a wide beam satellite antenna disposed on each GPS satellite. The wide beam antenna permits any GPS receiver having a line-of-sight to the GPS satellite to receive the navigational signal, and when the navigation signal from a sufficient number of GPS satellites has been acquired, the GPS receiver can determine its position via a precision clock and well-known triangulation techniques.
Because GPS signals are also used in military applications, countermeasures can be expected to be applied in an attempt to reduce the effectiveness of the GPS system. One such countermeasure is jamming. To increase the effectiveness of the GPS signals in a jamming environment, a steerable high gain antenna may be used to transmit high intensity GPS signals via spot beams to areas where needed.
One difficulty with this approach is that the high gain spot beam antenna is typically physically displaced from the wide beam antenna, and consequently, the phase center of each antenna is also displaced as well. This displacement is known as the “lever arm” between the antennas, and left uncorrected, can negatively affect the ability of the GPS receivers to determine their position. Without any correction, the lever arm between the wide beam antenna and a 7 meter diameter spot beam antenna can contribute up to 4.4 meters of user range error (URE). Depending on the GPS satellite constellation, this uncompensated URE can produce up to nine meters (RMS) of vertical (altitude) navigation error, which is a factor of 10 higher than the performance of the current GPS constellation of 0.9 meters (RMS). In civil aviation applications, such errors are sufficient to result in loss of life, and in military applications, they can result in increased collateral damage, and increased sortie and weapon consumption to perform the same mission.
Further exacerbating this problem is the fact that in order to maintain proper Sun and Earth pointing, the GPS satellites are required to perform attitude maneuvers. Such maneuvers can be very large, particularly about the yaw axis.
To achieve such high-accuracy navigation demanded from many missions, the GPS system must provide users with real-time corrections of the spot beam antennas phase center location relative to the Earth coverage antenna phase center, even while the spot beam antenna is moving to track specific terrestrial locations.
What is needed is an apparatus and method for computing a correction for lever arm related errors, and for incorporating this correction in navigation computations. The present invention satisfies these needs.